Allergic reaction can be categorized into type I allergic reaction, type II allergic reaction, type III allergic reaction and type IV allergic reaction, and their reaction principles are as follows: 1. Type I allergic reaction: In case of type I allergic reaction, when the allergen enters the organism, it will stimulate the B cells to produce IgE antibody, which will be adsorbed on the surface of basophil granulocytes and mast cells. If the same antigen invades the body again and combines with the IgE antibody in the body, it will cause an immune reaction. 2. Type II metaplasia: Type II metaplasia refers to the binding of antibodies to the antigenic components of the cell itself, which then causes lysis and death of the target cell through activation of the complement system. In addition, the combination of antibody and antigen will also promote the target cell surface antigen-bound IgG antibody through immunomodulation, which can promote the phagocytosis of target cells by macrophages. 3. Type III allergic reaction: Type III allergic reaction refers to the immune response that occurs after the antigen enters the body, and the antigen and antibody will form a complex that will be cleared by the body system. If the complexes formed are deposited in the tissues, they may cause tissue damage and immune complex disease. 4. Type IV allergic reaction: Type IV allergic reaction is mediated by T-cells. Generally, after the body is first exposed to an allergen, the T-cells in the body will be converted into sensitized lymphocytes, which will put the body in a state of sensitization. If the same allergen enters the body again and is recognized by the sensitized T-cells, an inflammatory reaction will occur, causing tissue necrosis. It is recommended that the organism needs to seek medical attention if an allergic reaction occurs, and the doctor will choose the appropriate measures for treatment depending on the type of allergic reaction.